This is a very interesting thread for me. I have been debating on rebuilding or replacing with like the 20HP Onan - maybe even going with the 24HP version.
But at the end of the day, the Onan is very old technology - not very fuel efficient and will be very difficult to maintain (they don't make this engine any more).
Something to remember on the weight is that you will eventually strip off much of the factory engine as you install / mount it. You will make your own baffling - you can remove flywheels since the prop is enough. You can even take off the starter, if you like. I therefore think that the new engines will not weigh much more than the original Onan. Please note that I have not verified this yet - if anyone has any actual weight data, please post it.
I have been considering the Kohler CH745. Let's compare it with the EH72 Low Profile UTV:
Max Power - 28 at 3600 vs 28.9 at 4000 RPM (26 at 3600)
Displacement - 725 cc vs 720 cc
Peak Torque - 42.7 vs 38.2 ft-lb
Bore - 83 vs 84 mm
Stroke - 67 vs 65 mm
Length - 14 vs 12.5 in
Width - 17.7 vs 18.8 in
Height - 26.5 vs 16.1 (note the Kohler includes the air filter)
Weight - 94 vs 101 lbs (note the Kohler includes the air filter)
The Kohler has a thrust bearing option - It looks relatively easy to install.
In summary, it looks like the Kohler has more power and weighs less.
My current concerns are that both of these engines are taller from the crank to the top than the original Onan. This means the engine cowling may be higher than the original, possibly blocking the view. I may want to lower the engine mount to compensate, but need to keep the prop out of the ground. I have resigned myself to re-fabricating te cowling, but I am not worried about that.
Another concern noted previously is weight. Again, I think that I can take enough weight off to make it work. The extra HP will make up for the extra ~10# weight, but the balance is a concern. I would want to move any possible weight to the rear of the plane such as battery, transponder, etc... It would be critical to keep the engine center of mass as far rearward as possible. On the original Q1 plans, the rear flywheel is actually inside of the cockpit (in the pie pan) to keep the weight of the Onan as far rea as possible. IF the balance is maintained, it should be fine - some wing loading calculations would be in order, though.
I agree that the electronic ignition and electronic fuel injection are real nice. I expect the fuel efficiency would increase dramatically as compared to the Onan, especially at the higher altitudes. The original design did not have any mixture control, so as you flew higher, you were wasting gas.
Remember more power means more heat to get rid of - baffeling needs to be well thought out and constructed. Also, both the Kohler and Subaru have O2 sensors. It doesn't take much leaded fuel to ruin them. So, you need to only use the (cheaper) autogas. Also, I would want to know what would happen if the O2 sensor fails - I believe the Kohler defaults to a standard mixture ratio.